Geopsy: Decimate time

Decimate time pop-up window.

Decimate time, reached from the Waveform menu, decreases the sampling frequency of signals in the active signal viewer. By clicking on Decimate time, a pop-up window appears requesting the decimation factor, i.e. the ratio between current sampling frequency and desired sampling frequency.

Decimate time is used to decrease the size of manipulated signals, when their sampling frequency is larger than 2 times the largest frequency of interest (Nyquist frequency). The decimation factor should be greater than 1, values lower than one correspond to oversampling that can be achieved using the Oversample tool. Moreover, this factor should be an integer value $p$. In order to decimate by a non-integer rational value ${\frac {p}{q}}$, first Over sample by $q$ and Decimate by $p$.

The decimation algorithm first applies a low-pass anti-aliasing filter in the frequency domain, in that case a tapered cosine filter of 20% width, with the cut-off frequency equal to half of the desired sampling frequency. Then, in the time domain, one sample is kept every $p$ samples (for a decimation factor equal to $p$.

Decimate time creates new temporary signals in the active viewer.

See also:

• Decimation (Wikipedia)
• Nyquist frequency (Wikipedia)